A known approach to microscopy utilising x-rays is projection x-ray microscopy, in which a focussed electron beam excites and thereby generates a spot x-ray source in a foil or other target. The object is placed in the divergent beam between the target and a photographic or other detection plate.
There have more recently been a number of proposals for using the electron beam of an electron microscope to excite a point source for x-ray microscopy. Integration of an x-ray tomography device directly into an electron microscope was proposed by Sasov, at J. Microscopy 147, 169, 179 (1987). Prototype x-ray tomography attachments for scanning electron microscopes using charge coupled device (CCD) detectors have been proposed in Cazaux et al, J. Microsc. Electron. 14, 263 (1989), Cazaux et al, J. Phys. (Paris) IV C7, 2099 (1993) and Cheng et al X-ray Microscopy III, ed. A Michette et al (Springer Berlin, 1992) page 184. Ferreira de Paiva et al (Rev. Sci. Instrum. 67(6), 2251 (June 1996) have developed and studied the performance of a microtomography system based on the Cazaux and Cheng proposals. Their arrangement was an adaptation of a commercially available electron microprobe and was able to produce images at around 10 μm resolution without requiring major alterations to the electron optical column. The authors concluded that a 1 μm resolution in tomography was feasible for their device. All system components and methods of interpretation of image intensity data in these works were based on the mechanism of absorption contrast.
A review article by W. Nixon concerning x-ray microscopy may be found in “X-rays: The First Hundred Years”, ed. A Michette & S. Pfauntsch, (Wiley, 1996, ISBN 0.471-96502-2), at pp. 43-60.
International patent publication WO 95/05725 disclosed various configurations and conditions suitable for differential phase-contrast imaging using hard x-rays. Other disclosures are to be found in Soviet patent 1402871 and in U.S. Pat. No. 5,319,694. Practical methods for carrying out hard x-ray phase contrast imaging are disclosed in international patent publication WO 96/31098 assigned to the present applicant. These methods preferably involve the use of microfocus x-ray sources, which could be polychromatic, and the use of appropriate distances between object and source and object and image plane.
Various mathematical and numerical methods for extracting the phase change of the x-ray wavefield at the exit plane from the object are disclosed in the aforementioned WO 96/31098, in Wilkins et al “Phase Contrast Imaging Using Polychromatic Hard X-rays” Nature (London) 384, 335 (1996) and in international patent publication WO 98/28950. The examples given in these references primarily related to macroscopic objects and features, and to self-contained conventional laboratory type x-ray sources well separated in space from the sample.
International patent publication WO 98/45853 discloses a sample cell arrangement especially useful for x-ray ultramicroscopy, in particular x-ray imaging, absorption and/or phase contrast, in the evacuated sample chamber of a scanning electron microscope. A target layer of the sample cell is activated by the SEM electron beam to direct an x-ray beam into the sample space of the cell. One embodiment described has multiple discrete micro-target spots irradiated by the electron beam, an advantageous arrangement in which the effective x-ray source size is determined by target dimensions and not necessarily by focal spot size of the electron microscope. Outstanding difficulties, however, are that the arrangement is very sensitive in two dimensions to e-beam/target alignment, and that background x-ray radiation can be quite substantial if the electron beam also strikes the target substrate.
In a bulk target the x-ray source size and shape is determined by the x-ray generation volume. Typically the x-ray source size for a bulk target is greater than 0.5 micron and so is unsuitable for x-ray sub-micron ultramicroscopy It is an object of the invention to provide an improved x-ray microtarget source that at least addresses one or more of these outstanding problems.
The inventors have appreciated that a target form known in atom probe field ion microscopy may be usefully adapted to the present application.